The present invention resulted in part from the observation that a standardized package of food is heated more quickly in certain microwave ovens than others. Foods do not appear to couple well with the microwave energy in certain ovens causing less heating and poorer results in certain ovens than in others. For example, popcorn may not pop very well in some ovens. In virtually all home microwave ovens, food is supported on a ceramic or glass shelf or false floor that is spaced an inch or two above the metal oven floor. The distance of the false floor above the metal floor of the oven varies from one manufacturer to another. In the course of developing the present invention, it was found that a standardized package of food was subjected to different heating conditions in different ovens. It was also determined that the amount of heating, i.e., the efficiency with which heat is induced into the food, appeared to be influenced by the height of the false floor from the microwave reflective metal floor of the oven.
It is a general objective of the present invention to provide, in its preferred form, a device that will assure more uniform heating of foods in a variety of microwave ovens of differing dimensions and will improve, speed up and generally facilitate the heating of foods but which is formed entirely from flexible or semiflexible packaging materials adapted to be withdrawn from a roll, printed, cut and formed like a conventional package so that little if any is added to the cost of an ordinary package.
A number of devices have been previously proposed to assist in the heating of foods in a microwave oven. For example, U.S. Pat. No. 4,013,798 describes a box which forms a shield supporting a metal tray above a reflective bottom layer. However, the box extends over the top of the food and prevents microwave energy from reaching it from the top. The shielding effect of the box together with the loss of microwave energy through holes in the bottom layer tend to prevent microwave energy from reaching the food. Microwave energy is reflected away from both the top and bottom of the surrounding box.
U.S. Pat. No. 3,835,280 proposes a microwave perturbating device composed of two heavy layers of plastic having between them two concentric rings of aluminum foil. The plastic layers are relatively expensive due to the large amount of resin needed as well as molding and other fabricating costs. Moreover, the small size and shape of the reflective surfaces as well as the large space between them make the metal rings a poor reflector of microwave energy.
U.S. Pat. No. 4,306,133 describes a microwave heater in which food held in an aluminum pan rests upon a heater formed from silicon carbide which is itself mounted above a reflective plate 19. During operation, the silicon carbide absorbs microwave energy and as it becomes hot the heat is transmitted by conduction to the food product through the aluminum pan.
A variety of prepared foods are now sold in the supermarket within a package specifically designed to contain the food while the package is heated within a microwave oven. To be successful, these packages must be composed of inexpensive, flexible or semiflexible packaging material such as paper, paperboard or foil capable of passing through conventional paper and paperboard converting equipment such as printers, sheet cutters and the like. One important object of the invention is to improve the heating of such packaged foods without adding much to their cost.
In the accomplishment of the foregoing and related advantages and objectives, the invention comprises the features hereinafter fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention by way of example, these being indicative, however, of but a few of the various ways in which the principles of the invention may be employed.